1. Field of the Invention
The invention relates to power supplies and, more particularly, to inrush surge current limit circuits. It is specifically concerned with automatic resetting of the inrush current limit circuit in response to a momentary drop of the input voltage to the power supply.
2. Description of the Prior Art
Power supplies generally include input filters to alleviate noise problems. The filter generally includes capacitive components which induce a large instantaneous transient current to the power supply when it is initially energized. The instantaneous transient current, known as the inrush current, can cause circuit breakers or fuses to operate and may damage the circuit components of the power supply.
The inrush current is normally limited to a safe value by inserting a large impedance in the input current path. The impedance may be switched out of the input path and a low impedance path substituted therefor after the initial inrush current transient has subsided. The substitution of the low impedance path may be performed manually or by an automatic system.
Manual switching systems are slow and insertion of the low impedance path is not related to the actual value of the input current. Prior automatic switching systems have operated on a fixed time delay or a time delay related to the stages of operation of the power supply which are unrelated to the actual magnitude or duration of the inrush current. This arrangement is inefficient since the input impedance dissipates power unnecessarily after the current inrush has ended. An additional disadvantage of these inrush current limiting arrangements is that should the input voltage to the power supply subsequently decrease, the inrush current limit circuit does not operate again to protect the power supply against the subsequent inrush current when the voltage level is restored.
It is, therefore, an object of the invention to limit the inrush current surge of a power supply for the exact duration of the inrush current.
It is another object of the invention to improve the efficiency of an inrush current limit circuit by bypassing the inrush current limit circuitry in response to a drop in the inrush current.
It is yet another object of the invention to reactivate the inrush current limit protection circuitry should the input voltage to the power supply decrease below a threshold value during operation of the circuit.